Thomas Aquinas

“ Slavery is contrary to marriage as regards the act to which marriage binds one party in relation to the other, because it prevents the free execution of that act; and again as regards the good of the offspring who become subject to the same condition by reason of the parent's slavery. Since, however, it is free to everyone to suffer detriment in that which is his due, if one of the parties knows the other to be a slave, the marriage is none the less valid. ”

“ Now just as nature seeks the preservation of the individual, so does it seek the preservation of the species by means of procreation; wherefore even as a slave is not so subject to his master as not to be at liberty to eat, sleep, and do such things as pertain to the needs of his body, and without which nature cannot be preserved, so he is not subject to him to the extent of being unable to marry freely, even without his master's knowledge or consent. ”

“ Yet it is not contrary to the second, because natural reason has this inclination, and nature has this desire---that everyone should be good; but from the fact that a person sins, nature has an inclination that he should be punished for his sin, and thus slavery was brought in as a punishment of sin. Nor is it unreasonable for a natural thing to be hindered by that which is unnatural in this way; for thus is marriage hindered by impotence of coition, which impotence is contrary to nature in the way mentioned. ”

“ Nothing prevents a thing being against nature as to the first intention of nature, and yet not against nature as to its second intention. Thus, as stated in De Coelo, ii, all corruption, defect, and old age are contrary to nature, because nature intends being and perfection, and yet they are not contrary to the second intention of nature, because nature, through being unable to preserve being in one thing, preserves it in another which is engendered of the other's corruption. And when nature is unable to bring a thing to a greater perfection it brings it to a lesser ”

“ Further, two favorable things outweigh one that is not favorable. Now marriage and freedom are favorable things and are contrary to slavery, which in law is not a favorable thing. Therefore such a slavery ought to be entirely annulled in marriage.
Objection 3: Further, in marriage husband and wife are on a par with one another. ”

“ Moreover, he who enters religion or receives orders binds himself to works that are superadded to natural works, and in which his master has power over him, but not in natural works to which a man binds himself by marriage. ”

“ The natural law requires punishment to be inflicted for guilt, and that no one should be punished who is not guilty; but the appointing of the punishment according to the circumstances of person and guilt belongs to positive law. ”

“ It is because the son derives honor from his father rather than from his mother that in the genealogies of Scripture, and according to common custom, children are named after their father rather than from their mother. But in matters relating to slavery they follow the mother by preference. ”

“ In such a case it is said that the master should be compelled not to sell the slave in such a way as to increase the weight of the marriage burden, especially since he is able to obtain anywhere a just price for his slave.
Reply to Objection 5: By entering religion or receiving orders a man is bound to the Divine service for all time; whereas a husband is bound to pay the debt to his wife not always, but at a fitting time ”